Nighthawk RAX43

I just bout a nighthawk rax43 and when ever I plug in the internet it doesn’t seem to connect but when I do this to the old router it work fine I looked up ways to fix this but they all say unplug modem I do not own a modem and a little confused about what to do please help and thank you. Also for a later topic when ever I plug a Ethernet cored in it doesn’t seem to work on either router if someone could help me with that it would be awesome too thank you